In an apparatus such as an oscilloscope or related equipment for precision testing or measurement of an electrical phenomenon, the phenomenon is ordinarily displayed as a visual plot of the instantaneous voltage of a wave as a function of time. In most cases, the time reference is supplied by the apparatus itself. For example, a sweep generator develops a sawtooth, or ramp, voltage which after being processed in a horizontal amplifier is applied to the horizontal deflection plates of the cathode-ray-tube (CRT), producing the horizontal sweep. When the voltage required to produce a linear unit of beam displacement (i.e., deflection factor) and the ramp voltage rate of rise are known quantities, the horizontal dimensions of any waveform traced on the CRT face plate can be converted to units of time.
If precise time measurements are to be made, it is essential that equal increments of horizontal distance represent equal increments of time. If the displacement of the electron beam is at all times proportional to the voltage applied to the horizontal deflection plates, it must follow that the sweep voltage change at a linear rate. Additionally, the electron beam must be returned to a quiescent position else, apparent positional shifts of the displacement of the electron beam occurs.
Thus, two important requirements for precision ramp generators are the stability of the baseline voltage prior to initiation of the ramp waveform and the lack of aberrations following initiation of the ramp.
Basically, all ramp generators for generating ramp voltages generate the ramp voltage by charging and discharging of a capacitor by a charging current. Previous schemes for baseline stabilization have limited the charging current to a particular valve thus requiring longer times for recovery from negative transients such as discharge overshoot. Start-up aberrations, due to the time interval to effectively switch current into the timing capacitor have been reduced by other schemes such as overdriving an emitter coupled amplifier; however, such schemes also produce aberrations caused by stray and device capacitances.